The operation of a cryogenic air separation plant requires that high boiling contaminants, such as water, carbon dioxide and hydrocarbons, be removed from the feed air before entering a column. Water and carbon dioxide will freeze at the cryogenic temperatures associated with the air separation. Hydrocarbons constitute a safety hazard if there is a potential for accumulation in a boiling oxygen pool.
Preferably adsorption prepurifiers are used to remove high boiling impurities from a feed air stream to a cryogenic air separation plant. Two types of such prepurifiers are employed: Thermal Swing Adsorption (TSA) and Pressure Swing Adsorption (PSA). As the names imply, TSA depends primarily on heat to drive the adsorbed contaminants off the adsorbent whereas PSA uses differences in absolute pressure within the adsorbent bed to cause the contaminants to desorb. Both are operated in a batchwise manner where feed air is passed through a clean adsorbent bed and the bed loads up with contaminants which are then removed in a desorption step. Typically, two or more adsorbent beds are used, with at least one bed in the adsorption mode and at least one bed in the desorption mode.
While such adsorption prepurifiers are excellent at removing carbon dioxide and water vapor from a feed air stream to a cryogenic air separation plant, they are typically not completely effective in removing hydrocarbons from the feed air stream. This requires that further measures be taken, such as the use of cold end gel traps and/or column liquid drains to ensure against hydrocarbon build up in oxygen-rich liquid within the column or columns of the cryogenic air separation plant. It would be very desirable to have a system which will improve the effectiveness of hydrocarbon removal from a feed air stream to a cryogenic air separation plant.
Accordingly, it is an object of this invention to provide a system which will enable more effective hydrocarbon removal from a feed air stream to a cryogenic air separation plant which employs adsorbent purification for prepurification of feed air to the plant.